Use datadir not datarootdir for dracut

The zfs dracut modules should be installed under the --datadir
not --datarootdir path.  This was just an oversight in the
original Makefile.am.

After this change %{_datadir} can now be set safely in the
zfs.spec file.  The 'make install' location is now consistent
with the location expected by the spec file.

dracut/90zfs/Makefile.am

@@ -1,4 +1,4 @@
-pkgdracutdir = $(datarootdir)/dracut/modules.d/90zfs
+pkgdracutdir = $(datadir)/dracut/modules.d/90zfs
 dist_pkgdracut_SCRIPTS = \
 	$(top_srcdir)/dracut/90zfs/90-zfs.rules \
 	$(top_srcdir)/dracut/90zfs/check \